The oceans have been the cradle for all life on Earth (as the earliest life began in the seas) and they were the refuge for life in times of dramatic changes in the climate.
The movement of ocean currents (and atmospheric gases) connects all life on Earth. The molecule of oxygen you just inhaled may have been emitted by a plant on the other side of the world. The carbon dioxide and water we breath out (and emit from the exhaust pipes of our automobiles) will be used to make more plants or phytoplankton somewhere else. The oceans also transport much tropical heat with slow moving currents of warm water, while undercurrents of cool water (carrying oxygen and nutrients) move in the opposite direction. These flow, across the Atlantic and Pacific, in endless conveyors that may take a thousand years.
The most well known example is the Gulf Stream (from the Gulf of Mexico) that keep Iceland, and most of northern Europe, much warmer than they would be otherwise.
About 71%, or 360 million square kilometres (km²), of the Earth's surface area is covered with seas and oceans. This provides the planet with considerable thermal inertia; which is why most of the planet does not freeze over at night.
The oceans absorb most of the solar radiation reaching the Earth and serve as giant heat reservoirs that moderate the climate noticeably near oceans.
Other notable ocean systems include El Niño and La Niña which occur every two to seven years and can last from several months to more than a year. El Niño is above normal sea surface temperatures in the central and eastern tropical Pacific Ocean whereas La Niña is characterized by cooler-than-normal sea surface temperatures in the same region.
El Niño weakens, or may even reverse, the direction of the trade winds (that normally blow westward across the Pacific) allowing warm water to move eastward. This significantly affects weather patterns around the world typically causing heavy rainfall and flooding in parts of South America and droughts in Southeast Asia and Australia.
El Niño also causes milder winters in northern parts of America, wetter conditions in the southern USA and drier weather in parts of Africa and Asia. All of which significantly affect crop yields and food production.
La Niña strengthens the prevailing trade winds blowing across the Pacific Ocean, pushing warm surface waters farther to the west and allowing cooler waters to rise to the surface in the central and eastern Pacific. This cooling of sea surface temperatures significantly affects global weather patterns.
La Niña typically causes below average temperatures in parts of South America and in the northern USA and Canada. Conversely, it can bring warmer conditions to the southern USA.
La Niña also increases hurricane and tropical storm activity in the Atlantic Ocean while decreasing those in the eastern Pacific Basin. This changes precipitation patterns that can also affect agriculture and food production.
From 1901 to 2018, the average global sea level rose between 1 and 2 mm per year. Between 2013 and 2022, this rate increased to 4.6 mm per year. Melting glaciers accounted for 21%, with Greenland accounting for 15% and Antarctica 8% but most of the increase was caused by the thermal expansion of ocean waters.
(Where sea ice is floating, it does not affect sea level when it melts).
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